The present invention relates to a power supply device, and more particularly, to a power supply device including a plurality of switching regulators which are synchronized with each other in their switching frequency.
Most electronic devices such as video cassette recorders (VCR) and video cameras, whether portable or installed on vehicles or the like, use batteries for driving power. But the terminal voltage of batteries may vary greatly when there occurs a sudden change in the load as a result of mode switching. To suppress these variations and stabilize the operation of the electronic devices, stabilizing power suppliers are currently used. These power supply devices use a switching regulator, and to provide an output voltage adaptive to more than one load, a plurality of switching regulators are necessary.
FIG. 1 shows a conventional separately excited switching regulator used in the power supply device contemplated by the present invention. A d.c. input Vi supplied across input terminals 2A and 2B has its ripple component removed in a capacitor 4 provided between the two input terminals. The input then is converted to an a.c. voltage in a switching transistor 6. The a.c. voltage is rectified in a d.c. reproducing circuit 14 composed of a diode 8, a choke coil 10 and a capacitor 12. Subsequently, the d.c. voltage is drawn from output terminals 16A and 16B as a stabilized d.c. output Vo.
For drawing a voltage-divided bias from the d.c. output voltage Vo, resistors 18 and 20 are connected in series between terminals 16A and 16B. The divided output developing at the junction between the resistors 18 and 20 is fed to an error amplifier 22 where it is compared with a reference voltage provided by a reference power supply 24. The detected difference is fed to the non-inverting input terminal of a comparator 26, and an output comprising triangular waves as shown in FIG. 2A from a reference wave generator circuit 28 is supplied to the inverting input terminal of the comparator 26. In FIG. 2A, the varying output produced from the error amplifier 22 is denoted by Ve. Given the two inputs, the comparator 26 performs a pulse width modulating operation and generates pulses whose width is controlled by the varying output V.sub.e as shown in FIG. 2B. These pulses are fed to a transistor 30 as a control input, and in response to the switching operation of the transistor 30, a pulsive drive current flows into the switching transistor 6 through a resistor 32 and, thus, the transistor 6 conducts a switching operation. A bias resistor 34 is connected between the emitter and base of the switching transistor 6.
If a plurality of switching regulators having the above-described circuit configuration are assembled to make up a power supply device, a reference wave generator is necessary for each switching regulator. This first of all makes the overall configuration of the power supply device complex. In addition, if the oscillating frequencies of the individual generator circuits are not in synchronism with each other, serious beating may occur, and this phenomenon must be prevented by all means because it produces a noise problem and can cause incorrect operation during the oscillation and signal processing in internal circuits of VCR or video cameras. The same problem occurs if the regulators used are of a self-excited switching type having respective oscillation control units.